Benchmark (surveying)

An Ordnance Survey cut mark in the UK

This article is about the use of bench marks in surveying, for other uses see Benchmark.

The term bench mark, or benchmark, originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in future. These marks were usually indicated with a chiseled arrow below the horizontal line.

The term is generally applied to any item used to mark a point as an elevation reference. Frequently, bronze or aluminum disks are set in stone or concrete, or on rods driven deeply into the earth to provide a stable elevation point.

The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark, a point with a precisely known relationship to the level datum of the area, typically mean sea level. The position and height of each benchmark is shown on large-scale maps.

The terms "height" and "elevation" are often used interchangeably, but in many jurisdictions they have specific meanings; "height" commonly refers to a local or relative difference in the vertical (such as the height of a building), whereas "elevation" refers to the difference from a nominated reference surface (such as sea-level, or a mathematical/geodetic concept near sea level known as the geoid). Elevation may be specified as normal height (above a reference ellipsoid), orthometric height, or dynamic height which have slightly different definitions.

Other types of survey marks

An Ordnance Survey flush bracket

Triangulation points, also known as trig points, are marks with a precisely established horizontal position. These points may be marked by disks similar to benchmark disks, but set horizontally, and are also sometimes used as elevation benchmarks. Prominent features on buildings such as the tip of a church spire or a chimney stack are also used as reference points for triangulation. In the United Kingdom, triangulation points are often set in large concrete markers, which as well as functioning as a triangulation point, have a benchmark set into the side. With the increasing use of GPS and electronic distance measuring devices, the same techniques and equipment are used to fix the horizontal and vertical position of a survey marker at the same moment, and therefore the marks are usually regarded as "fixed in three dimensions".

Agencies responsible for benchmarks

Benchmarks are typically placed ("monumented") by a government agency or private survey firm, and many governments maintain a register of these marks so that the records are available to all. These records are usually in the form of a geographically searchable database (computer or map-based), with links to sketches, diagrams, photos of the marks, and any other technical details.

Government agencies that place and maintain records of benchmarks include:

• Canada
  • Natural Resources Canada Geodetic Survey Division (History of the Geodetic Survey Division)
• France
  • Institut Géographique National (IGN on Wiki FR)
• Italy
  • Istituto Geografico Militare - Servizio Geodetico
• Japan
  • Geographical Survey Institute (GSI)
• New Zealand
  • Land Information New Zealand
• Spain
  • Instituto Geográfico Nacional (IGN)
• United Kingdom
  • Ordnance Survey
• United States
  • The National Geodetic Survey (NGS; formerly U.S. Coast & Geodetic Survey)
  • The United States Geological Survey (USGS)
  • The United States Army Corps of Engineers (USACE)

Image gallery

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  C&GS benchmark disk in the United States.

• 

  Bench mark at Saint Goussaud, Limousin, France, by Institut Géographique National.

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  This USGS survey marker is designed to be a standard nail to be used by the USGS to mark high-watermarks, set reference points, set bench marks, set reference marks, and turning points for levels. This nail is designed to be recovered at later dates without any question that the nail was set by the USGS.

• 

  City of Toronto benchmark disk in Canada.

See also

• Benchmarking—a recreational activity in which participants search for benchmarks using a handheld GPS receiver.
• Geoid

External links

• UK benchmark database
• Detailed description of the different types of UK benchmark
• US NGS Benchmarks overlaid on a Google Map
• Benchmarks and levelling points